Risks to Small Populations

How the demographic and genetic structures of wild plant populations respond to (sub) population size, patterns of disturbance and habitat conditions

Small populations risk losing demographic viability and genetic diversity particularly as climates change and habitats become more fragmented. Can we predict these risks? How can we best sustain populations?

Conservation biologists seek to discover mechanisms that threaten the persistence of native species and communities in order to devise and implement effective conservation strategies. Our lab continues to investigate the demographic and genetic structure of wild plant populations in relation to patterns of disturbance and spatial aspects of habitat structure. Often, our goal is to understand how habitat features and meta-population dynamics affect the persistence of rare plants like Pedicularis furbishiae (Gawler et al. 1987), Aconitum novaboracense, and Cirsium pitcherii. Owen Boyle examined how spatially patchy fire dynamics affect metapopulation dynamics and genetic structure in the endangered Florida scrub endemic, Polygonella basiramia in order to estimate its minimum viable metapopulation size and arrangement.

We also use isozyme and DNA markers to assess population genetic structure in relation to population size and environmental conditions (e.g., in Pedicularis – Waller et al. 1988; and the subject of a masters thesis by Michael Stevens, Populus tremuloides – Stevens et al. 1999). Former postdoc Jeff Dole and former MS student Andy Bersch studied RAPD variation among populations of wild rice (Zizania palustris) in northern Wisconsin (under contract with the Ojibway Indian Tribes). Catherine Woodward is comparing reproductive success and pollen flow among continuous and fragmented populations of tropical trees with different modes of pollination. Such information leads, in turn, to a better understanding of how the size and arrangement of habitat patches and patterns of disturbance affect genetic diversity and ultimately long-term persistence.

The regional persistence of species subject to local population colonization and extinction necessarily depends on how landscape features and disturbance affect metapopulation dynamics. Here, we characterize the metapopulation structure and short-term dynamics of Polygonella basiramia. This rare, short-lived perennial herb is endemic to Florida scrublands and lacks a seed bank. Fires create the open sand gaps within a shrub matrix that support this species but also kill established plants. Thus, persistence depends on frequent colonization of unoccupied gaps. We are monitoring population dynamics within and among 1204 gaps distributed among 19 shrub patches. Considerable subpopulation turnover is evident at the gap level with rates of gap extinction exceeding rates of colonization in the first year. Whether declines in overall abundance continue is likely to depend on patterns of disturbance and regional stochasticity in this dynamic landscape. Polygonella is more likely to occupy larger and less isolated gaps, demonstrating that landscape features and disturbance strongly affect metapopulation dynamics. Because Polygonella basiramia displays characteristics, occupancy patterns, and turnover dynamics consistent with metapopulation theory, it represents a model system for studying plant metapopulations.

A rare episode of regeneration of aspen (Populus tremuloides Michx.) by seeds occurred in Yellowstone National Park, (YNP), Wyoming, USA, following extensive fires that occurred in 1988. We sampled 410 aspen seedlings from 23 local populations distributed widely across YNP in 1997 to determine how genetic diversity varies with elevation, substrate, plant competition, ungulate browsing, and geographic location. We employed 132 randomly amplified polymorphic DNA (RAPD) markers based on six primers to show genetic relationships within and among the postfire aspen seedling populations. Measures of genetic variation, including estimates of percent polymorphic loci, expected heterozygosity, and NeiÌs FST, indicated that most of the variation occurred within rather than among local populations. There was no indication of geographic differentiation among sampled populations based on hierarchical estimates of Nei’s FST, neighbor-joining, or correlations between genetic distance and geographic distance. Even genetically distant populations shared nearly 90% of the same markers, Within plots, the amount of genetic variation decreased slightly in response to increased percent vegetative cover, mean seedling basal diameter, and mean seedling height. Geologic substrate, density of lodgepole pine (Pinus contorta var. latifolia Dougl.) seedlings, browsing intensity, and elevation were not significantly related to levels of genetic variation within the seedling plots. These data suggest that genetic variation and geographic structure among seedling populations may occur over time as the transition from seedling-dominated stands to clone-dominated stands occurs. (Stevens et al. 1999)